Process for producing n-phosphonomethyl glycine triesters

ABSTRACT

This disclosure relates to an electrolytic process for producing N-phosphonomethyl glycine esters by the electrolysis of the tetra esters of N-phosphonomethylimino diacetic acid. In accordance with this disclosure, a solution of a tetra ester of Nphosphonomethylimino diacetic acid dissolved in a suitable solvent containing an electrolyte is subjected to an electromotive force or electric current in an electrolytic cell to be electrolytically oxidized to a triester of Nphosphonomethyl glycine. The triester of N-phosphonomethyl glycine produced can be hydrolyzed to the free acid and its salt derivatives which are useful as post-emergent herbicides.

United States Patent [191 Wagenknecht Jan.7,1975

[ PROCESS FOR PRODUCING N-PHOSPHONOMETHYL GLYCINE TRIESTERS [75]Inventor: John H. Wagenknecht, Kirkwood,

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: Nov. 7, 1973 [21] Appl. No.: 413,476

[52] US. Cl 204/59 R, 204/74, 204/78, 204/79, 260/941, 260/968 [51] Int.Cl. C07b 29/06, C07f 9/40 [58] Field of Search 204/59 R, 78, 7974;260/941, 968

[56] References Cited UNITED STATES PATENTS 2,978,392 4/1961 MacLean etal. 204/59 R 3,137,705 6/1964 Prelog et al 204/79 X 3,649,484 3/l972Prager 204/79 Primary Examiner-F. C. Edmundson Attorney, Agent, orFirm-William T. Black; Donald W. Peterson [57] ABSTRACT This disclosurerelates to an electrolytic process for producing N-phosphonomethylglycine esters by the electrolysis of the tetra esters of N-phosphonomethylimino diacetic acid. In accordance with this disclosure,a solution of a tetra ester of N- phosphonomethylimino diacetic aciddissolved in a suitable solvent containing an electrolyte is subjectedto an electromotive force or electric current in an electrolytic cell tobe electrolytically oxidized to a triester of N-phosphonomethyl glycine.The triester of N-phosphonomethyl glycine produced can be hydrolyzed tothe free acid and its salt derivatives which are useful as post-emergentherbicides.

5 Claims, No Drawings PROCESS FOR PRODUCING N-PHOSPHONOMETHYL GLYCINETRIESTERS This invention relates to a method of producing N-phosphonomethyl glycine triester thereof by the electrolytic oxidationof the tetra esters of N- (phosphonomethyl) imino-diacetic acid. Moreparticularly, this invention relates to the production of N-phosphonomethyl glycine triesters by the electrolysis of tetra esters ofN-(phosphonomethyl) imino-diacetic acid in a suitable solvent mediumcontaining a supporting electrolyte.

Reference is made to copending application Ser. No. 385,932, filed Aug.6, 1973, which discloses a method for producing N-phosphonomethylglycine by electrolysis of N-organo-N-phosphonomethyl amino acetic acidcompounds.

In accordance with the process of this invention, an organic solventelectrolytic solution of the tetra ester of N-phosphonomethyliminodiacetic acid is charged into an electrolytic cell fitted with an anodeand a cathode and an electromotive force or electric current impressedupon the cell whereby the tetra ester of N- phosphonomethyliminodiacetic acid is electrolytically oxidized to yield the triester ofN-phosphonomethyl glycine, as the principal product.

In a preferred method of conducting the process of this invention, a 5%to 20% solution of tetra ester of N- phosphonomethylimino diacetic aciddissolved in acetonitrile containing a dissolved supporting electrolyteis charged into an electrolytic cell maintained at a temperature of fromC. or less to 100 C. or more and having noble metal, graphite or carbonelectrodes. An electric current is then impressed on the cell byconnecting the anode and cathode to a proper source of direct currentwith controls to maintain the current density at between .01 and 100ma/cm for a time sufficient to oxidize the tetra ester of N-phosphonomethylimino diacetic acid to the triester of N-phosphonomethylglycine.

The resultant reaction solution is then vacuum evaporated to remove thesolvent and reaction by-products. The triester residue is then dissolvedin water, hydrolyzed and recovered as a N-phosphonomethyl glycine byrecrystallization upon the cooling of the water solution.

The concentration of the tetra ester of N- phosphonomethylimino diaceticacid employed in the process of this invention is not critical and islimited only by the solubility of the starting material in theparticular solvent employed. Thus, for example, although concentrationsas low as 0.01% by weight in the solvent can be employed, for reasons ofefficiency and economy, it is preferred to employ concentrations of fromabout 5 to about 30% by weight, or even higher, of the tetra ester ofN-phosphonomethylimino diacetic acid in the solvent containingsupporting electrolyte.

The temperature at which the process of the instant invention isconducted is not narrowly critical and can range from as low as 0 C. toas high as 110 C. or even higher if a pressure cell is employed. Thetemperature employed is a function of the solvent boiling point,freezing point, pressure and solubility of reactants and reactionproducts. As is apparent to those skilled in the art, at lowertemperatures a very dilute solution or a suspension must be employedsince the solubility of the tetra ester of N-phosphonomethyliminodiacetic acid starting material is lower at lower temperatures.

The process of the instant invention can be conducted at atmosphericpressure, super atmospheric pressures and subatmospheric pressures. Forreasons of economy and ease of construction of the equipment employed inthe process ofthis invention, it is preferred to conduct this process atapproximately atmospheric pressure.

The type of electrolytic cell employed in the process of this inventionis not critical. The cell can consist of a glass container having one ormore anodes and cathodes connected to a source of direct current, suchas a battery and the like or a source of low frequence alternatingcurrent. The cell can also consist of the two electrodes separated by aninsulator such as a rubber or other non-conducting gasket.

The current densities employed in the process of-this invention canrange from as low as 1 milliampere per square centimeter (malcm to 60 ormore ma/cm In general, it is preferred to employ current densities offrom about I to about 10 for best yields of the desired triester ofN-phosphonomethyl glycine. At higher current densities, the electrolyticefficiency of the cell is decreased. At the higher current densitiesthere are also undesirable side reactions, such as electrolysis of thesolvent and decomposition of the desired product into desirableby-products.

The electrodes, i.e., the anode and cathode, employed in the process ofthis invention can be constructed of a wide variety of materials andcombinations of materials. Thus, for example, the anodes may beconstructed of any conductive substance, such as lead, graphite, leadoxide, lead sulfate, carbon in various forms, platinum, various metaloxides such as manganese dioxide, copper oxide, nickel oxide and thelike, and can be in many different forms such as gauze, solids, porous,etc. Other electrode materials are less preferred since they corroderapidly and their ions contaminate the electrolyte, thereby renderingthe isolation of the product more expensive and difficult.

The cathodes can also be of any conductive substance such as copper,lead, platinum, palladium, lead oxide, graphite, carbon and the like. Itis preferred to employ a noble metal such as palladium or platinum orvarious forms of graphite, carbon or glassy carbon as the electrodematerials employed in the process of this invention.

The enabling electrolytes which can be employed to render the solventmedium conducting include the metal perchlorates, fluoroborates,acetates, hexafluoro phosphate and the like. The only limitation on theenabling electrolyte being employed is that it dissolves in the solvent,that it ionizes in the solvent and that it is not oxidized at thepotential of the oxidation of the tetra ester of N-phosphonomethylglycine in the specific solvent being employed. Specific examples ofsuch enabling electrolytes are salts such as ammonium hexafluorophosphate, ammonium fluoroborate and the a]- kali or alkaline earthmetal salts such as sodium, potassium or rubidium hexafluoro phosphate,sodium fluoroborate, tetramethylammonium fluoroborate,tetraethylammonium fluoroborate, tetramethylammonium ethylsulfate,tetraethylammonium ethylsulfate, trimethylammonium fluoroborate,trimethylammonium hexafluorophosphate, tetramethylammoniumtoluenesulfonate, tetraethylammonium toluenesulfonate dimethylammoniumfluoroborate, diethylammonium perchlorate, tetrapropylammoniumperchlorate, lithium perchlorate, tetraethylammonium acetate and thelike.

In the process of this invention, a solvent is essential. The solventmust be one in which the tetra ester of N- phosphonomethylimino diaceticacid is soluble and also in which the supporting electrolyte is solubleso that the solution is conductive.

Illustrative of the solvents that can be employed in the process of thisinvention are nitriles such as acetonitrile, propionitrile,benzonitrile, etc.; nitro compounds such as nitromethane, nitroethane,etc; halogenated hydrocarbons such as methylene chloride, ethylenechloride, etc; and cyclic ethers such as tetrahydrofuran, and the etherssuch as ethylene glycol dimethyl ether, diethylene glycol dimethyl'etherand mixtures of the above solvents with each other and with aliphaticalcohols, etc.

It is, of course, apparent to those skilled in the art that the time ofreaction is variable and is determined by variables such as currentdensity, electrode area concentration and volume of the reactionsolution.

The triesters of N-phosphonomethyl glycine product of the process of thepresent invention is recovered from the reaction solution byconventional techniques known to .those skilled in the art, such as byextraction and recrystallization, centrifugation, concentration and thelike. The triesters can by hydrolyzed with an acid such as dilutehydrochloric acid to yield N- phosphonomethyl glycine which is useful asa herbicide.

The hydrolysis reaction solution can be vacuum evaporated to eliminatethe water, acid, and alcohol by-product. The solid which remains can bedissolved in water and then cooled to precipitate the N- phosphonomethylglycine, which is recovered by filtration.

The tetra esters of N-phosphonomethylimino diacetic acid useful in theprocess of this invention are those having the general formula o o omoOR \ll /PCH2N\ /O O CHzC wherein R, R, R" and R" are each independentlymonovalent hydrocarbon radicals containing from one to 12 carbon atoms,halogenated monovalent hydrocarbon radicals, and hydrocarbonoxyhydrocarbon groups containing from one to four oxygen atomsinterconnecting the hydrocarbon moieties.

Illustrative of. the monovalent hydrocarbon radicals represented by R,R, R" and R' are alkyl groups of the formula C l-I such as methyl,ethylpropyl, butylhexyl, octyl, decyl, dodecyl and their isomers, etc;alkenyl groups of the formula C l-I wherein a is as previously defined,such as ethenyl, propenyl, butenyl, octenyl, dodecenyl and theirisomers, etc; aryl groups containing six through carbon atoms such asphenyl, tolyl, xylyl, ethylphenyl, diethylphenyl and the like; aralkylgroups such as benzyl, phenylethyl, phenylpropyl, dimethylphenylpropyl,dimethylphenylbutyl and the like; and the halogenated derivativesthereof containing up to three halogen atoms.

By the term halogen as employed herein is meant fluorine, chlorine,bromine and iodine.

Illustrative of the hydrocarbonoxy hydrocarbon groups represented by R,R, R" and R' are those of the formula RO-(R O),,,R wherein R is alkyleneor alkoxy alkylene of not more than eight carbon atoms; R is alkylene ofnot more than four carbon atoms; R is selected from the group consistingof alkyl and alkenyl of not more than six carbon atoms; and m is aninteger from 0 to 2. Illustrative of the groups represented by R'O [RO]m R are alkoxyalkyl, alkenoxyalkyl, alkoxyalkoxyalkyl,alkenoxyalkoxyalkyl, dialkoxyalkyl, alkenoxy(alkoxy)alkyl,alkenoxyalkoxy(alkoxy)alkyl and alkoxyalkoxy(alkoxy)alkenyl such as2-methoxyethyl, 4-ethoxy-2-methylbutyl, 2- ethoxyethyl, 2-propoxypropyl,4-methoxybutyl, 4-methoxy-2-ethylbutyl, 2-butoxybutyl, 2- allyloxyethyl,Z-butenoxyethyl, 4-butenoxybutyl, 2-(2-methoxyethoxy)ethyl,2-(2-ethoxy-ethoxy) ethyl, 2-(3-methoxypropoxy)propyl,2-(3-allyloxypropoxy)- ethyl, 2-(2-butenoxyethoxy )ethyl,2,4-dimethoxybutyl, Z-ethoxypropyl, 2,4-diethoxybutyl,2-methoxy-4-allyloxybutyl, l-ethoxy-2-propenoxyethyl, 4-( 2-allyloxyethoxy)2-methoxybutyl, 2-(4- methoxybutoxy)ethyl,2-(2-methoxyethoxy)butyl and the like.

The following example serves to further illustrate the process of thisinvention. In the example, all parts are by weight unless otherwisespecifically set forth.

EXAMPLE 1 This example was conducted in a glass frit dividedelectrolytic cell. The anolyte consisted of l.l8g. of thetetraethylester of N-phosphonomethylimino diacetic acid dissolved infifty milliliters of acetonitrile containing 0.2m.ammoniumhexafluorophosphate. The catholyte was 0.2m. NH, PF in acetonitrile. Theanode was platinum foil, the cathode platinum, and a saturated calomelreference electrode was in the anode compartment.

The electrolysis was conducted at +1 .6v vs. saturated calomelelectrode. The initial current was 30ma and after 5 hours had dropped toless than 3ma and 0.002 Faradays of electricity had been passed. Theanolyte was concentrated on a rotary evaporator. A portion of theresidue was hydrolyzed in 20% hydrochloric acid by refluxing for 1 hour.Nuclear'magnetic spectral analysis indicated that the residue was a 2 to1 mixture of N-phosphonomethylimino diacetic acid and N- phosphonomethylglycine indicating that a 33% conversion of the tetra ester to triesterhad been accomplished.

What is claimed is:

1. A process for producing a triester of N- phosphonomethyl glycinewhich comprises subjecting a solution of a tetra ester ofN-phosphonomethylimino diacetic acid in an organic solvent containing asupporting electrolyte to a direct electric current at a current densityof about 0.0] to about ma/cm whereby said tetra ester is oxidized to thetriester, said solvent being one in which the tetra ester and thesupporting electrolyte is soluble.

2. A process as claimed in claim 1 wherein the tetra ester is a tetraalkyl ester.

3. A process of claim 2 wherein the solvent is acetonitrile.

4. A process of claim 3 wherein the supporting electrolyte is ammoniumhexafluorophosphate.

5. A process of claim 3. wherein the tetra ester is tetraethyl-N-phosphonomethylimino diacetate.

1. A PROCESS FOR PRODUCING A TRIESTER OF N-PHOSPHONOMETHYL GLYCINE WHICHCOMPRISES SUBJECTING A SOLUTION OF A TETRA ESTER OFN-PHOSPHONOMETHYLIMINO DIACETIC ACID IN AN ORGANIC SOLVENT CONTAINING ASUPPORTING ELECTROLYTE TO A DIRECT ELECTRIC CURRENT AT A CURRENT DENSITYOF ABOUT 0.01 TO ABOUT 100 MA/CM2 WHEREBY SAID TETRA ESTER IS OXIDIZEDTO THE TRIESTER, SAID SOLVENT BEING ONE IN WHICH THE TETRA ESTER AND THESUPPORTING ELECTROLYTE IS SOLUBLE.
 2. A process as claimed in claim 1wherein the tetra ester is a tetra alkyl ester.
 3. A process of claim 2wherein the solvent is acetonitrile.
 4. A process of claim 3 wherein thesupporting electrolyte is ammonium hexafluorophosphate.
 5. A process ofclaim 3 wherein the tetra ester is tetra ethyl-N-phosphonomethyliminodiacetate.